Quadcopter capturing aerial imagery for crop monitoring. UAS used for motion picture filming. Model aircraft for hobby use. 

No, Part 107 requires a Remote Pilot certificate. Yes, as a Student Pilot they can. Yes, under the direct supervision of a remote Pilot in Command. 

Continue the mission, airplanes are not allowed to fly lower than 500 feet AGL unless around an airport. Give way and stay clear from the crop duster.   Continue the mission, ...

Visual observer.  Person manipulating the controls. Remote PIC.

the Remote PIC.  the controlling agency. Air Traffic Control.

They can maintain line of sight and without loss of control.  They briefed each other before the flight. They are in radio contact the entire time.

Remote Pilot Certificate, Registration Card, and a copy of the Completion Certificate for the Recurrent Training.  Remote Pilot Certificate and your logbook. Remote Pilot Certificate and Registration Card.